This invention relates to processes for the conversion of coal to a clean fuel, that is, a fuel which is substantially free of the mineral components normally found in coal.
In particular, the invention relates to coal liquefaction processes wherein a solvent (hereinafter sometimes called liquefaction solvent) is present during the liquefaction of the coal. Liquefaction may be achieved by hydrogenation, depolymerization, extraction, etc. The liquefaction solvent, which is generally coal-derived, may function as solvent for the coal or for the products, or both. It may also play a reactive role, for instance, in the depolymerization of the coal molecules. Examples of such coal liquefaction processes are described in U.S. Pat. Nos. 3,018,242; 3,117,921; 3,143,489; 3,158,561; 3,523,886; Re. 25,770; and 3,321,393.
The primary product of such coal liquefaction processes is a mixture of liquid and undissolved solids. Some gas is generally also produced. The liquid is a solution of coal liquefaction products dissolved in the liquefaction solvent. Some of the undissolved solids may be readily separated from the liquid by conventional solids-liquids separation processes such as filtration, centrifugation, sedimentation, hydroclones, etc. However, a large part of the undissolved solids (the amount being a function of the particular coal and the particular liquefaction treatment) appears as extremely finely divided particles of the order of ten microns or less in size. These particles are rich in mineral matter normally found in all coals. Upon combustion of the fuel which contains them, they form ash.
Efficient separation of such finely divided particles from the liquid in which they are suspended can either not be accomplished at all or at impractically low rates by the usual mechanical separation techniques at ordinary temperatures (i.e. by filtration, centrifugation, settling, or hydroclones) because of the extremely fine state of subdivision of the solid particles and because of the high viscosity of the liquid. Separation is improved by operation at elevated temperatures due to a rapid decrease in liquid viscosity, as well as an increase in the density differential between liquid and solid. Even at these elevated temperatures and reduced viscosities, the conventional separation techniques may be only partially effective.
The prior art offers many solutions to the problem of separation described above. Those which are pertinent to the present invention are those which use a precipitating solvent (sometimes called an anti-solvent) to effect agglomeration of the suspended solids, thereby permitting use of conventional solids-liquid separation techniques. Illustrative of the prior art pertinent to such agglomeration are the following U.S. Pat. Nos. 2,060,447; 2,631,982; 2,774,716; 2,871,181, 2,964,460; 2,989,458; 3,010,893; 3,018,241, 3,275,546; 3,519,553; 3,607,716; 3,607,717; 3,607,718; 3,607,719; 3,685,814; 3,642,608; and 3,687,837.
The most pertinent patent considered in connection with the preparation of this application is U.S. Pat. No. 3,791,956, patented Feb. 12, 1974 to Gorin, Kulik and Lebowitz.
The primary object of the present invention is maximum recovery of a product which is substantially free of mineral matter and undissolved coal.